The present invention relates to new liquid crystal monomers and polymers. In particular it describes compounds for use in liquid crystal mixtures and in liquid crystal devices such as liquid crystal displays (LCDs).
Monomeric and polymeric liquid crystals are being developed for use in flat screen televisions and other display devices. For such applications it is desirable to have an LCD that has a fast switching time, is resilient to impact or shock, can operate over a wide temperature range and has good optical properties. In general smectic liquid crystals have faster switching times than nematic liquid crystals, and antiferroelectric smectic liquid crystals have comparable switching times to ferroelectric liquid crystals. Although sinectic LCDs have fast switching times, they generally exhibit poor shock resistance. This problem may be overcome by using smectic liquid crystal polymers having a polymer backbone to which side chain mesogenic groups are bonded. In these polymers it is the mesogenic side groups that display orientational order in the liquid crystal phases.
A considerable amount of research has been undertaken in liquid crystalline polymers to control their properties by polymerising monomers with systematically different structures. With mesogenic side chain polymers this has been achieved conventionally by making small variations in the structure of the mesogenic group. Choosing different polymer backbones and varying the number of methylene units in the alkyl spacer between the mesogenic group and the backbone, also influences the properties. Although polymeric smectic liquid crystals exhibit good shock resistance, their mesogenic groups often exhibit poor alignment. Such poor alignment has an adverse effect upon the optical properties and temperature range of the liquid crystal. For example, polyacrylates have a mesogenic group attaxched to every second atom in the polymer backbone, and this close spacing constricts movement of the mesogenic groups resulting in relatively poor alignment. A further example is provided by polyoxetane liquid crystals having the mesogenic groups on every fourth atom of the polymer backbone, as is shown in the diagram below. 
This increased spacing and the greater flexibility of the backbone results in liquid crystal polyoxetanies having improved alignment properties. However a number of problems still remain, including slow switching speeeds and small temperature ranges.
In addition to the problems discussed above, polymeric LCDs can also suffer from relatively high melting points. To overcome this problem, polymeric liquid crystals can be mixed with monomeric liquid crystals to suppress the melting point. The use of liquid crystal mixtures has the further advantage that it allows other properties such as pitch to be optimised.
The invention solves these problems by the provision of new liquid crystal monomers and polymers, liquid crystal mixtures, and liquid crystal devices.
According to one aspect, the present invention provides a compound of formula (I): 
wherein X, X1, X2 are independently selected from straight or branched chain C1-16 alkyl, halogen and H;
 is any suitable mesogenic group,
Z=single covalent bond, oxygen CO2, OCO;
n=1-20; Y=oxygen, CO2, OCO, CH2, CHOH, CH2O
m=3-10000
the mesogenic group is defined from the general structure II:
"Parenopenst"Axe2x80x94W1"Parenclosest""Parenopenst"Bxe2x80x94W2"Parenclosest"0.1"Parenopenst"D"Parenclosest"Q
where A, B, and D are independently selected from: 
where W1 and W2 are independently selected from a single covalent bond, COO, OCO, CH2CH2, CH2O, OCH2, O;
Q is selected from:
CN, Halogen, R, OR, COOR, CF3, lactate derivatives, where R may be chiral, straight or branched chain alkyl and may include from 1-16 carbon atoms and including where one or more non-adjacent CH2 groups may be substituted by CH(CN), CH(CF3), CHF CH(Cl), CH(CH3);
the substituents on the phenyl and cyclohexyl rings indicate that at least one substituent may be present on the rings specified and up to four substituents present on the phenyl rings and up to ten substituents present on the cyclohexyl rings.
The structure of the compounds having formula (I) allows the mesogenic groups to align easily and to exhibit good switching properties. The compounds having formula (I) are suitable for display devices and and are particularly suitable for plastic display devices. The compounds having formula (I) exhibit Smectic C phases over a wide temperature range. The compounds of formula (I) can exhibit antiferroelectric Smectic C phases, provided that the R group is chiral.
Preferably: W1 is single covalent bond.
More preferably:
X is CH3, X1 and X2 are both H, Z is oxygen, n=6-12, Y=CH2O,
A is: 
W1 is a single covalent bond
B is: 
W1 is xe2x80x94COOxe2x80x94,
D is: 
and Q is xe2x80x94COOR.
Yet more preferably Q is: 
According to another aspect, the present invention provides compounds of formula (III): 
wherein X, X1, X2 are independently selected from straight or branched chain C1-16 alkyl, halogen and H;
 is any suitable mesogenic group:
Z=single covalent bond, oxygen CO2, OCO;
n=1-20; Y=oxygen, CO2, OCO, CH2, CHOH, CH2O
the mesogenic group is defined from the general structure II
Preferably W1 is a single covalent bond.
More preferably:
X is CH3 X1 and X2 are both H, Z is oxygen, n=6-12, Y=CH2O,
A is: 
W1 is a single covalent bond,
B is: 
W2 is xe2x80x94COOxe2x80x94,
D is: 
and Q is xe2x80x94COOR.
Yet more preferably Q is: 
According to a further aspect, the present invention provides a method of making an electro-optic device comprising one or more materials of formula III comprising the steps of:
forming a cell comprising two cell walls spaced apart, the walls inner surfaces having formed thereon electrode structures,
providing a mixture comprising a monomer material and a cationic initiator,
introducing the mixture between the cell walls,
polymerising the mixture.
There may also be present in the mixture a radical photoinitiator.
Preferably at least one wall is surface treated to provide liquid crystal alignment.
The monomer material may be aligned before polymerisation and/or the polymer may be aligned after polymerisation. The monomer may be present in any of the known liquid crystal phases including nematic, cholesteric or smectic.
Preferably the polymerisation is carried out under UV light and/or in the presence of additional heat or an electric field.
According to an alternative aspect, the present invention provides a liquid crystal device which comprises two spaced cell walls each bearing electrode structures and treated oil at least one facing surface with an alignment layer, and a layer of liquid crystal material enclosed between the cell walls, characterised in that the layer of liquid crystal material comprises a material of the formula I and/or III.
According to a further aspect of the invention provides a liquid crystal material having optically active properties comprising at least one compound according to the formula I. Advantageously the liquid crystal material comprises both the compound having formula I and the compound having the formula III.
According to a yet further aspect, the invention provides a liquid crystal material comprising a compound according to the formula III.